Bug fix for Option[Array[Byte]] on PostgreSQL. Closes issue #28.

New test case in DataTypeTest.testByteArrayOption. It turns out that
MS Access (using the standard JdbcOdbcDriver) also has problems with
this type but there is no known work-around for setting a LONGBINARY
value to NULL, so we add BasicProfile.capabilities.setByteArrayNull for
this feature and disable it in AccessDriver.